Modern-day communication and data processing equipment circuitry utilizes solid state integrated circuit technology to transmit and control data and switching information. Circuits utilizing solid state integrated circuit technology are highly susceptible to variations in electric power from some desired standard. The power to energize these circuits can be readily derived from commercial AC power. The signal consistency of commercial AC power, however, is unreliable due to user demand and other extraneous considerations causing power level fluctuations. These signal variations can take the form of blackouts, brownouts, or transient interruptions or surges. A blackout is a condition in which the power source fails completely. A brownout is a substandard signal condition wherein the output voltage of the power source is significantly reduced. Transient interruptions and surges are conditions due to momentary disturbances which alter the continuity of the power signal supplied by the power source.
Signal variations such as described above can significantly alter stored data and switching signals in the communication equipment and may, in some instances, damage the integrated circuitry contained therein. The most dangerous failure situation is the transient interruption or surge which may cause undetected errors because the failure is not readily apparent. Erroneous data or control signals can be generated and resulting damage occur which is not immediately detectable or obvious. These transient interruptions may be due to lightning faults or the operation of heavy electrical equipment in the nearby vicinity.
As is apparent from the foregoing, communications circuitry utilizing solid state integrated circuit technology requires a very reliable power source. This circuitry generally cannot tolerate voltage excursions greater than .+-.10 percent for longer than a duration of approximately one-half cycle of operation. Hence, if power variations in excess of these limits are allowed to occur, it can bring about costly shutdowns, disrupted communications, erroneous computations, and possibly damaged circuitry.
To counteract the susceptibility of the communications circuitry to the above-described power signal variations, uninterruptable power supplies (UPS) are utilized. A UPS guarantees the continuity of power regardless of the performance of the primary commercial AC power source basically relied upon.
A commonly used UPS is the continuous type power supply in which a rectifier charger, powered by a primary commercial AC power source, continuously energizes a subsequent inverter circuit. The inverter operates continuously to supply output power. A battery is floated across the output of the rectifier charger to supply the necessary DC power to the inverter should the primary commercial AC power source vary significantly or fail. This continuous type UPS arrangement advantageously uses very few circuit components. However, the reliability of such a system is no greater than the reliability of the inverter circuit which must operate continuously to supply output power. In addition, such an arrangement is inefficient because the overall efficiency is no greater than the combined efficiency of the inverter and the rectifier charger, resulting in a total efficiency lower than the individual efficiency of either component.
Another type of UPS operates in a parallel continuous arrangement in which two independent sources, a primary commercial AC power source and a secondary reserve AC power source, operate continuously in parallel. These two AC signals are combined and supplied to the load continuously. Should one of the signals fail, the remaining operative source takes over to supply the full load signal to the circuitry to be energized. An example of this type of UPS is disclosed by R. E. Kuba in U.S. Pat. No. 3,398,292, issued Aug. 20, 1968. Kuba discloses a ferroresonant transformer arrangement to accept power from two discrete independent but synchronized AC power sources. The ferroresonant transformer arrangement combines the two input power signals and supplies the resultant signal to a single output load. The two input primary windings of the transformer are connected to the primary AC power source and the reserve AC power source, respectively, and are isolated from each other by high reluctance magnetic shunts. Both inputs, however, simultaneously share the output load. If one of the input power sources fails, the other input power source operates to supply the total load power. Since the two input power sources operate continuously and simultaneously, each input power source operates at less than its highest efficiency.
A more efficient UPS arrangement is the transfer type. In the transfer type arrangement, the full power to the load is normally supplied by a commercial AC power source. When the commercial AC power source is operating normally, the reserve power supply, which is usually a battery-powered static inverter, does not supply power to the load but operates in a standby mode. If the AC commercial power fails, the inverter is connected to supply the power to the load and the commercial AC power source is disconnected from the load. The power supply sources operate independently in the alternative to supply power to the load. The advantage of this system is that each power supply can be designed to operate at its maximum efficiency. In addition, there is the protection of full redundancy to cover a failure of either the primary or the reserve power source.
To supply the complete protection necessary to protect communications or data processing equipment against transient failure of the primary power source, the transfer or switching from one power source to another must be almost instantaneous and very reliable. The reserve power source must supply power to the load almost instantly upon failure of the commercial AC power source. When commercial AC power is restored, the UPS must transfer the load smoothly and rapidly from the reserve power source to the commercial AC power source.
A transfer type UPS is disclosed by R. Schumacher et al in U.S. Pat. No. 3,229,111, issued Jan. 11, 1966. Schumacher discloses an AC power system with a standby reserve power source. The output power is normally supplied from an AC commercial power source to a load. The continuity of output power is assured by a reserve power source which is maintained in a standby condition. The reserve power source comprises a battery-driven static inverter. The inverter generates a signal controlled to have a predetermined fixed phase lag with respect to the AC line signal. The fixed phase lag relationship is established so that when the commercial AC power source is functioning normally it supplies substantially all the load power requirements. If the commercial AC power signal fails, the output power is supplied by the battery-driven static inverter.
Another embodiment of a transfer type UPS is disclosed by R. S. Jamieson in U.S. Pat. No. 3,348,060, issued Oct. 17, 1967. Jamieson discloses a no-break power supply which utilizes a continuously operating standby reserve power source to back up a commercial AC power source. The standby reserve power source is synchronized in frequency and correlated in phase with the commercial AC power source. The standby reserve power source operates continuously during normal operation of the commercial AC power source. The standby reserve power source does not, however, transfer any significant power to the load during normal operation of the commercial AC power. Upon failure of the commercial AC power source the load is transferred to the standby reserve power source. The power to operate the standby reserve power source is derived from the commercial AC power source. This power is derived from a battery which is floated on a charger powered by the commercial AC power source. Should the commercial AC power fail, the inverter operates to supply the output load power.
Another transfer type UPS is disclosed by E. C. Rhyne in U.S. Pat. No. 3,339,082, issued Aug. 29, 1967. Rhyne discloses a UPS wherein a static inverter circuit and a commercial AC power source are connected in parallel to a ferroresonant system to guarantee continuous power to a load to be powered by an AC signal. As long as the commercial AC power source is functioning properly, power to the load is preferably drawn from the commercial AC power source rather than from the inverter. If the commercial AC power source fails, power to the load is supplied by the static inverter.
The above-described UPS arrangements which switch from one power source to another power source advantageously provide redundancy through transfer from a failed power source to an operating power source. These UPS arrangements, however, generally rely upon complicated transformer and static switch arrangements to transfer from one power source to another. Some of the transformer arrangements do not provide isolation between the input and output of the power supplies. In addition, they do not simultaneously permit the reserve power source or static inverter to idle losslessly when the commercial AC power source is operating normally and, further, subsequently permit the reserve power source and the commercial AC power source to operate simultaneously and to share the load power in the event of a brownout, where the commercial AC power source signal is degraded. This lack of versatility limits the maximum attainable efficiency of the power supply.
In view of the foregoing, it is desirable to have a power supply which operates from a primary or commercial AC power source and a reserve power source in which the reserve power source is power-demand responsive to supply uninterrupted power to a load. The goals of an improved UPS system are to allow highly efficient load sharing between a primary power source and a reserve power source and, upon demand, to provide almost instantaneous backup power to the commercial AC primary power source with a minimum of inverter power consumption.